Apparatus for measuring the tensile strength of green sand



May 11, 1965 w. PATTERSON E'rAL. 3,182,493

APPARATUS FOR MEASURING THE TENSILE STRENTH OF GREEN SAND Filed NOV. 26, 1962 Fig/ Fig? VII/ll United States Patent C) 3,182,493 APPARATUS FR MEASURING THE TENSELE STRENGTH GF GREEN SAND Wilhelm Patterson, Intzestrasse 5, and Dietmar Bosnisch, Haselsteig 11, both of Aachen, Germany Filed Nov. 26, 1962, Ser. No. 240,059 Claims priority, application Germany, .lune 19, i962, P 29,648 Claims. (Ci. 73-95) This invention relates to a method of blending and/or compacting green moulding sand and to apparatus for measuring the tensile strength of the green sand. In order to permit the mechanical properties of natural and synthetic green moulding sands to be judged it is the general practice in foundries uniformly to compact the moulding sand to a predetermined density in a laboratory apparatus and then to determine its compressive as well as its shear strength. The results thus obtained provide a basis for the comparative assessment of the behaviour of sands in the casting mould. The tensile strength of green sands has likewise been the subject of measurement in the laboratory, but inferences relating to the behaviour of the sand in the mould have never been based on such tests. 4In the compacted sand of a casting mould each grain of quartz is held in position by the adhesive power of the binder envelope surrounding the same. This cannot be determined otherwise than by means of a tensile test because the results of compressive tests are largely overlaid by the frictional forces engendered when the grains are relatively displaced. In a compressive test the shape of the grain and the nature of the grain surface play a considerable part and they may cause assessments of the cohesive power of the clay mineral used and of the probable behaviour of the sand in the mould to be far from correct. Moreover, in the performance of compressive tests the surface of fracture is larger than would be the case in the practical conditions in whichthe sand is packed in the foundry because of the generation of pressure cones. The conditions -which obtain in such a test therefore differ from those which arise in actual practice.

Sand moulds are rarely critically stressed by compression. The critical stresses are far more frequently tensile. Insuiiicient tensile strength is therefore much more likely to lead to failures of the mould and to faults in the casting than the lack of compressive strength of the sand. Furthermore, moist sands tend to stick to the pattern material so that sand of low tensile strength may easily be the cause of moulds being damaged during pattern withdrawal. Many parts of a mould, especially parts of the cope, are subject to tensile stress which may be further accentuated by expansion of the sand when exposed to the casting heat. Fracture of the mould is then likewise due to insufficient tensile strength of the green moulding sand.

The object of the invention is to blend and/ or compact moulding materials in such a way that the casting mould lwill have the best possible mechanical strength. Another object of the invention is to eliminate the drawbacks of known methods and apparatus employed for determining the mechanical properties of moulding materials and to provide both a method and apparatus which will permit a test to be performed under conditions which resemble those obtaining in the actual mould. For solving these problems it is first proposed by the present invention to make additions to the moulding sand in such quantities 'and/ or to compact the sand in such a way that its tensile strength in the finished mould has its maximum possible value. However, the invention is not confined to blending and/ or compacting the moulding sands with a view of obtaining maximum tensile strengths. It is also proposed to blend the moulding sands in such a way that increasing SZAQB Patented May ll, 1965 ice compaction produces a rapid rise of the green tensile strength to a maximum when the degree of compaction is still low. It has been found that the tensile strength of a sand is about 10% of its compressive strength and that this relation is not the same in every sand but considerably depends upon the quality of the clay binder. The ratio of tensile to compressive strength rises with the quality of the clay binder, provided all the other conditions of the test with respect to compaction, type of sand, preparation and so forth are equal. For instance, the ratio of tensile to compressive strength in the case of bentonites is roughly about 0.1, in the case of beutonitic clays it is roughly between 0.06 and 0.1, in the case of clays of natural sands, such as kaolinite, ellite, glauconite and so forth, it is under 0.06. This means that sands of like compressive strength have different tensile strengths, depending upon the quality of the clay used as a binder, and that the conventional compressive test provides no conclusive and reliable evidence for assessing the behaviour of the moulding sand when it is moulded and exposed to the casting heat. It can therefore frequently be observed that with increasing wear compressive strength is maintained for .a considerable time, whereas the tensile strength falls very rapidly. Hence a sand, though possessing adequate compressive strength, may have to be classified as being poor. Moreover, some additives which improve compressive strength have the effect of lowering the tensile strength. Such additives are for instance quartz meal. Other additives, such as swelling binders improve both compressive and tensile strength. In order to provide a good moulding sand it is therefore proposed to blend the same in such a way that the ratio of tensile to compressive strength of the sand is approximately 1:10.

It has also been found that the 4tensile strength of the green sand depends upon various factors and that with increasing compaction it first rises and after reaching a maximum falls again. This phenomenon may be explained by the fact that increasing compaction presses the originally loose bulk of the grains closer and closer together, thereby iirst improving cohesion bet-Ween the grains. However, when a certain degree of comp-action has been exceeded cohesion becomes looser again with a reduction in binding ability, and hence the tensile strength of the sand becomes less. On the other hand, increasing compaction continuously raises compressive strength which does not pass through a maximum and which does not fall again. The curve obtained by a compressive test cannot therefore be a criterion for the optimum degree of compaction at which a satisfactory casting mould will be produced. Hence, the tensile-compressive strength ratio reveals itself as a novel and important factor in assessing the quality of a moulding sand. The higher the ratio, the fewer diiculties are likely in use to arise. This ratio can be measured with particular precision and very easily determined because it is not related to any praticular moisture content of the sand. Compressive strength varies considerably within a range of a few tenths of a percent of moisture and the reliable adjustment of a reference -water content, for instance of the optimum moulding content, is a matter of considerable difficulty. On the -other hand, two different blends of sand cannot be compared by reference to their compressive strength at the same water content, say at 3% water content, because every sand requires different contents of waterfor optimum mouldability according to composition and the number of components contained in the sand. It has further 'been recognised that increasing compaction by ramming, tamping, vibration or pressing leads to increasing compressive strengths, whereas the tensile strength reaches a peak and then falls again. The maximum is reached at a different degree of compaction in each sand Aand a sand may well lose its tensile strength though not its compressive strength by overcompaction. It has also been recognised that for varying sand composition the optimum green tensile strength is reached at different degrees of compaction.

It has further been found that rising content of water causes the green tensile strength to fall in exactly the same way as it does with increasing compaction, as has been described. Though despite a high water content the green tensile strength will still pass through a maximum on compression and then fall, the water content has no effect on the absence lof a compressive strength peak. Yet another discovery is that with rising clay content and rising compaction the green tensile and compressive strengths increase considerably and that the maximum of green tensile strength is more quickly reached at higher clay contents, the curve falling away more gently after the maximum than otherwise.

The grain size also improves green tensile and compressive strengths because increasing grain size reduces the total surface of all quartz grains in a specimen so that the binder envelope on each grain becomes thicker. The maximum of green tensile strength is clearly displaced to the left with increasing grain size. In other words, the maximum of green tensile strength is reached at less compaction. It has also been found that the angularity of the quartz grains, i.e. the ratio of the surface of the quartz grain to the surface of a sphere of like mass and density determines the tensile strength of the green sand. If the maximum is to be reached at low compaction it is therefore an advantage to use a round grain. However, the size of the grain should also be chosen with a view to avoiding the risk of penetration.

For determining the tensile strength of a moulding sand, particularly of a green sand, the invention proposes that part of the sand which is being compacted should be contained in a preferably shallow cylindrical measuring head with an open end and the tensile strength measured by withdrawing the measuring head and determining the force required for detaching the sand contained in the head from the rest of the sand. This proposal permits the tensile strength of a sample of sand to be measured or `of the sand already contained in the tlask of a mould. Different moulding machines have in fact been found to give very different tensile strengths for the same sand and it is therefore advisable to measure the tensile strength of a sand in the mould after compaction by the moulding machine. To this end the invention further proposes to build the measuring head into the casting mould and t-o surround the same with a ring which remains in the mould when the casting head is withdrawn, so that the measuring instrument is rmly supported on the soft sand. Preferably the measuring head should be forwardly coned and have a polished outer surface to reduce the adhesion of the outside of the head to the sand suiciently to be negligible.

For measuring and applying the tensile force to the sample of sand the inventi-on proposes to apply pressure in a closed pressurisable chamber axially to a precompressed spring until the resistance of the spring is overcome, the end of the spring facing the pressure chamber being `then attached to the measuring head and the pressure in the chamber slowly relaxed, and to determine the balance of pressure remaining in the chamber at which the moulding sand inside the head separates from the rest of the sand.

Preferably the testing instrument may comprise a hand operable pump with a pressure gauge and a pressure chamber with a piston having a piston rod surrounded by a helical spring of which one end bears on the piston, whereas the other end bears on a retaining plate axially slidable on the rod, the bottom end Iof the piston rod being connectable to the measuring head, whereas the retaining plate which is slidable on the piston rod is supported youtside the instrument by a part which is stationary with respect to the measuring head.

At its lower free end the piston rod carries a coupling element for the measuring head and preferably this may take the form of a magnet. Conveniently the magnet is liftable by a hand bail and magnetically attaches itself to the measuring head when the instrument is placed into position for performing the test. The movement of the piston rod and/ or of the measuring head can be read on a dial. Moreover, conveniently the connection between the piston rod and the magnet or like coupling element may include a bearing which is laterally displaceable and forms a universal joint, thus allowing for slight differences in position of the measuring head, especially when this has been directly inserted into a mould.

At several levels the inside of the measuring head has undercut faces for the reliable retention of the moulding sand inside the measuring head. The side of the measuring head facing the magnet is socket-shaped and provided with coned centering faces. Moreover, the magnet is similarly provided with coned centering faces to facilitate central insertion into the socket of the measuring head. If desired, the measuring head might be replaced by one or more pins or like members interconnected by a plate to permit the stickiness of a moulding sand and the extracting force to be measured for withdrawing the pattern. This applies both to the sand and to varying materials used for making the patterns.

According to yet another feature of the invention the testing instrument is provided with a tripod with coned feet provided at a given distance above their pointed extremities with a collar which bears on the member supporting the measuring head when the instrument is in position for testing.

In order that the invention may be more readily under stood reference will now be made to the acompanying drawings which illustrate the invention by showing em bodiments of the proposed apparatus.

FlG. 1 is an axial elevation of an embodiment of apparatus for determining the tensile strength according to the invention,

FIG. 2 is a vertical section of the supporting base for mounting the compaction cylinder under the ram, and

FIG. 3 is the measuring head inserted into a mould.

The test apparatus illustrated in FIG. 1 comprises a pressure chamber 10 connectable at 11 with a hand pump, not shown and with a gauge 11'. The pressure chamber is formed by two circular plates 12 and 13 with suitable cylindrical recesses. A rubber rolling membrane 14 or diaphragm is clamped between the two plates for sealing the chamber from a piston 15 which is axially slidably movable in a cylindrical cavity. Piston 15 is attached to a piston rod 16 which projects from the pressure charnber. Firmly aixed to piston rod 16 are a plate 17 and a guide member 18 made of plastic. In the illustrated embodiment these two members are threadedly attached to the piston rod.

The bottom end of the piston rod is screwed into a cylindrical holder 19 which is formed with a hole 20 for insertion thereinto of a key or a pin for rotating the holder 19 and thereby lengthening or shortening the overall length of the piston rod 6 including the holder. Recessed into the bottom of holder 19 is a downwardly open cylindrical cavity 21 with an internal screw thread permitting the cavity to be closed by a screw cap 22 provided with a central core 23 of ample diameter for the passage therethrough with all-round clearance of a pin 24, the forward end of said pin being screwed into a ball 25 which rests in a socket in a further ring 26 having an external diameter less than the internal diameter of the cylindrical cavity in holder 19, thus permitting the ring to move radially and, in conjunction with the ball, to suspend pin 24 with universal freedom of movement. The bottom end of pin 24 is screwed into a magnet 27 with the interposition of a laterally projecting circular disc 28 which can be raised and lowered together with the magnet by means of a bail 29 having two extensions 30, 31 and supporting members 32, 33 attached to said extensions. The cylindrical holder 19 which is attached to the piston rod and to the magnet is embraced by a plate 34 which is axially slidably movable on holder 19 as well as in the cylindrical bore of a plate-shaped guide member 35 which is itself secured to columns 36 and 37. The drawing shows only two such columns 36 and 37, though in fact there may be three or four. The columns are provided with threads which permit the elevational position of the plate-shaped guide 35 to be adjusted by nuts which work on the threads of the columns'.

The lower ends of the columns are let into a baseplate 3S which has a central circular opening with cylindrical guiding faces 39 for guiding the magnet 27. Underneath the baseplate 33 at equiangular 120 intervals around the circumference of the plate are three pointed legs 4t) with radial bearing faces 41. These guide the apparatus centrally into the upper annular iiange 42 of the compaction cylinder 43, permitting magnet 27 to apply itself centrally to the measuring head 44. To this end the latter has a recessed socket with centering edges facing the magnet 27. The measuring head 44 is itself centrally located inside the compaction cylinder 43. Moreover, at two dierent levels the measuring head 44 is formed with undercut internal faces 45 and 46.

The tensile test is performed by iirst filling the pressure chamber with compressed air until the power of the spring 47 held between the two retaining plates 17 and 34 is just balanced. This can be recognised by an axial motion of piston rod 16 indicated on a dial not specially shown in FIG. 1. The instrument is then picked up by the bail 29 and placed on the annular flange 42. Bail 29 is then lowered to bring magnet 427 into contact with the measuring head 44. The compressed air is released from chamber 10 unt-il the power of the spring prevails and pulls magnet 27 together with the measuring head 44 upwards. The pressure existing in chamber 10 when the moulding sand in the measuring head separates from the sand in the compaction cylinder in the plane of separation 43 is now read on the pressure gauge 11. By reference to a calibration curve which relates the air pressure to the tensile s-trength of the sand at the moment of rupture the tensile strength of the tested sand can be determined.

FIG. 2 shows the position of the measuring head 44 in the compaction cylinder 43 prior to the sand being lled into and compacted inside the cylinder. The compaction cylinder is mounted on a bedplate 49 in the ramming machine and held in position by a push and turn joint. The measuring head is lifted and pressed against the end face of the compaction cylinder 43 by a member 50, which is threadedly mounted in the bed plate 49 to engage and clamp the measuring head 44 in place in the cylinder 43. When the moulding sand has been compacted the further connection between the compaction cylinder and the measuring head is maintained by the compacted sand itself. The cylinder can therefore be removed from bedplate 49 and the screw plate and the tensile test can be performed as already described by reference to FIG. 1.

FIG. 3 shows the manner in which the measuring head 44 can be inserted into a ask 51 which is only partly shown in the drawing. The measuring head is embraced by a ring 52 upon which the testing instrument can be placed, and which is retained in the sand by its annular groove 54 which forms a key when the measuring head is extracted from the flask. Moreover, during compaction, a ring 53 is provided which centralises the measuring head in a ring 52, and which is removed before the measuring instrument is placed over the head.

What we claim is:

l. An apparatus for measuring the tensile strength of molding sand, said apparatus comprising a measuring head having a recess for receiving and retaining ia portion of a body of sand to be tested, a pull rod, coupling means for releasably connecting one end of said pull rod and said head, a piston on the opposite end of said rod, a cylinder slidably receiving said piston, a compression spring for biasing said piston inwardly of said cylinder, means for admitting air pressure to said cylinder and a pressure indicator connected to said cylinder, whereby upon admission of air pressure to said cylinder to move said piston and rod outwardly and coupling of said rod to said head, subsequent release of pressure from said cylinder will permit said spring to move said piston and rod inwardly to move said head and separate the sand therein from the body of sand, the air pressure at the instant of separation of said sand providing an indication of the tensile strength of the sand.

2. An apparatus as defined in claim l, and including a plate through which said pull rod is movable, one end of said spring engaging said plate and the opposite end engaging said piston, means for adjusting the position of said plate axially with respect to said pull rod and means attached to said plate for engaging a supporting surface.

3. An apparatus as defined in claim 1, in which said coupling means comprises a magnet connected to said pull rod and engageable with said head for releasably coupling said pull rod and head together and a hand engaging bail attached to said magnet.

4. An apparatus as defined in claim 1, in which the recess in said measuring head is cylindrical and includes an outer part and an inner part with the outer part being of greater diameter .than the inner plart and with the walls of said outer and inner parts converging outwardly and toward the center to provide under cut recesses for receiving and retaining said portion of a body of sand. j

5. An apparatus as dened in claim 1, and including a tubular cylinder for receiving said body of sand, an enlarged portion at one end of said cylinder treminating in a peripheral flange and providing an annularrecess for receiving said measuring head, said annular recess terminating at the inner end in a shoulder for engaging said measuring head, means for holding said measuring head in engagement with said shoulder and for supporting said cylinder and measuring head while filling said cylinder and measuring head with sand to be tested, said lastnamed means comprising a bed plate for engaging said peripheral iiange and measuring head to support the same in inverted position and cooperating means on said bed plate and ange providing a bayonet joint between said cylinder and said bed plate.

6. An apparatus as defined in claim 2, and including a tubular cylinder for receiving said body of sand, an enlarged portion at one end of said cylinder terminating in a peripheral flange and providing an annular recess for receiving said measuring head, said annular recess .terminating at the inner end in a shoulder for engaging said measuring head, said means for engaging a supporting surface comprising conical studs projecting from said plate for engaging the outer edge of said annular recess to center said plate with respect to said cylinder and shoulders on said studs for engaging said flange to support said plate thereon.

7. An apparatus as defined in claim 2, and including a ring to be embedded in the body of sand surrounding said measuring head and radially spaced therefrom, said ring serving to provide the supporting surface for engagement by the means attached to said plate.

8. An apparatus as dened in claim 3, in which the connection between said pull rod and said magnet is in the form of a joint providing for swinging movement and lateral displacement of said magnet with respect to said pull rod.

9. An apparatus as deiined in claim 3, in which said measuring head is provided with a conical recess for receiving said magnet, said magnet being tapered to cooperate with said conical recess in centering said magnet with respect to said head.

10. An apparatus as defined in claim 5, in which a member is threadedly received in said bed plate for en- '2,959,957 11/60 Smith et ai 73-95 gaging said measuring head to clamp the same in engage- 3,036,459 5/ 62 Kendall 73--95 ment With said shoulder. 3,057,182 10/ 62 Hackett 73-95 X References Cited by the Examiner 5 FORETGN PATENTS UNITED STATES PATENTS 2,481 5/08 Great Britain.

2,275,341 3/42 Brabender 73-169 l 2,419,711 4/47 Dillon 7.3 103 RICHARD C. QUEISSER, Plzmafy E/wmmel. 2,789,688 4/57 Stinson 73--1()3 JOSEPH P. STRIZAK, Examiner.

2,904,994 9/59 Claxton 73-92 10 

1. AN APPARATUS FOR MEASURING THE TENSILE STRENGTH OF MOLDING SAND, SAID APPARATUS COMPRISING A MEASURING HEAD HAVING A RECESS FOR RECEIVING AND RETAINING A PORTION OF A BODY OF SAND TO BE TESTED, A PULL ROD COUPLING MEANS FOR RELEASABLY CONNECTING ONE END OF SAID PULL ROD AND SAID HEAD, A PISTON ON THE OPPOSITE END OF SAID ROD, A CYLINDER SLIDABLY RECEIVING SAID PISTON, A COMPRESION SPRING FOR BIASING SAID PISTON INWARDLY OF SAID CYLINDER, MEANS FOR ADMITTING AIR PRESSURE TO SAID CYLINDER AND A PRESSURE INDICATOR CONNECTED TO SAID CYLINDER, WHEREBY UPON ADMISSION OF AIR PRESSURE TO SAID CYLINDER TO MOVE SAID PISTON AND ROD OUTWARDLY AND COUPLING OF SAID ROD TO SAID HEAD, SUBSEQUENT RELEASE OF PRESSURE FROM SAID CYLINDER WILL PERMIT SAID SPRING TO MOVE SAID PISTON AND ROD INWARDLY TO MOVE SAID HEAD AND SEPARATE THE SAND THEREIN FROM THE BODY OF SAND, THE AIR PRESSURE AT THE INSTANT OF SEPARATION OF SAID SAND PROVIDING AN INDICATION OF THE TENSILE STRENGTH OF THE SAND. 